Ya i'm not sure where the hang up is.

In this macro the null is to make it drop down to the next line, right? (i could also put "/castsequence shadowfiend,," also couldn't i)?

/target party3
/cast Power Infusion
/assist party1
/castsequence Shadowfiend, null
/castsequence Devouring Plague, Shadow Word: Pain

Issue with above is that shadowfiend is on a 5 minute cooldown, meaning that it will only cast devouring plague & shadow word: pain once each every 5 minutes

the other one, with reset=combat still relies on a 5minute cooldown.

perhaps i need to try

/target party3
/cast Power Infusion
/assist party1
/castshadowfiend

/castsequence Devouring Plague,,

/cast shadow word: pain